Purification of polyolefins



United States Patent 3,415,800 PURIFICATION OF POLYOLEFINS Abraham Kutner, Newark, Del., assignor to Hercules Incorporated, a corporation of Delaware No Drawing. Filed Mar. 2, 1964, Ser. No. 348,788 7 Claims. (Cl. 260-93.7)

ABSTRACT OF THE DISCLOSURE In the process where olefins are polymerized by means of transition metal catalysts and the catalyst residues are deactivated by treatment of the polymer slurry with low molecular weight alkanols, addition of a small amount of a monoor dialkyl orthophosphate to the treated slurry reduces the sensitivity of the slurry to air and moisture, permitting the remainder of the working procedure to be accomplished without nitrogen blanketing.

The process is usually carried out by x to an inert organic diluent that is liquid under the reaction conditions and passing the Q ethylene or other olefin into the catalyst mixture at atmospheric or lightly elevated pressure and at room temperature or slightly above room temperature. When an olefin is so polymerized, a highly crystalline stereoregular polymer is obtainable that has many industrial uses. In the process, the polymer which is insoluble in the reaction medium precipitates out and can be separated from the diluent by any of the usual means such as filtration, centrifugation, etc. However, when no steps are taken to purify the polymer, it normally contains a large quantity of catalyst residues which adversely affect its color, its stability, and its electrical properties, and render the polymer corrosive to metal. Hence, it has been necessary to devise methods for purifying these polymers to rid them from the catalyst residues that are inherently present at the completion of the polymerization process.

Numerous methods for purifying the stereoregular polyolefins have been suggested. One such method comprises Washing the polymer, after separation from the polymerization diluent, with mineral acids as, for example, methanolic hydrochloric acid, aqueous solutions of nitric acid, etc. This type of treatment gives very pure polymers but requires the use of considerable quantities of expensive reagents.

By another process that has been suggested, the polyolefin, while still slurried in the polymerization diluent, is treated with an alkanol to solubilize the catalyst residues and the alkanol-containing slurry is then washed with an aqueous liquid to extract the catalyst residues from the slurry. The diluent phase, containing the polymer, and the aqueous phase, containing the catalyst residues, are then separated by decantation. The latter method generally gives excellent results in that the amount of metal residues contained in the purified polymer is generally reduced to about 200 parts per million. There is still need, however, for a process for purifying these polyolefins that reduces the amount of catalyst residues in the polymers to an even lower amount.

One of the most effective processes known for removing the metallic catalyst residues from these polyolefin slurries is described in British Patent 840,233. In this process the polymer slurry is treated with an alcohol to solubilize the catalyst residues, as in the process decribed immediately above; solid polymer is then separated from the slurry, e.g., by filtration; and finally, the polymer is washed with a liquid hydrocarbon. This process, when properly carried out, results in extremely low levels of catalyst residues in the polymer. However, it is necessary, in order to obtain the full benefits of the process, to effect the separation of polymer in an inert atmosphere such as nitrogen, because contacting of the slurry with air and moisture causes precipitation of the metal catalyst residues and consequently higher metal content in the polymer. The maintenance of an inert atmosphere during the separation of the polymer is cumbersome and costly. The art, therefore, would very .much welcome any improvement in the method which would permit handling the alcohol-treated slurry in the presence of air.

The present invention is directed to an improvement in the process of the aforesaid British patent, which improvement comprises adding to the alcohol-treated slurry from about 0.1 to 2% by weight (based on the weight of diluent) of an anionic, oil-soluble phosphate ester having the formula in which R represents hydrogen or a hydrocarbon radical of 4 to 22 carbon atoms and R represents a hydrocarbon radical of 4 to 22 carbon atoms. By virtue of this improvement, the slurry is surprisingly rendered insensitive to air and moisture, and further steps in the recovery and purification of the polymer can be taken without regard to the maintenance of an inert atmosphere. In fact, even when the slurry is allowed to come in contact with air of exceptionally high humidity, it has nevertheless been possible on some trials to prepare polymers containing as little as 30 parts per million of total metal.

In accordance with an optional feature of the invention, there is added to the slurry, either prior to or following the addition of phosphate ester, a small amount of alkylene oxide, such as propylene oxide, the purpose of which is to neutralize any hydrochloric acid present in the slurry and thereby reduce its corrosivity to the apparatus employed in the work-up. The addition of alkylene oxide is, of course, not essential, but is desirable since it makes possible the use of less expensive apparatus.

In order to avoid any premature oxidation of polymer which might occur upon filtration of the slurry, especially at elevated temperatures, it may sometimes be desirable to add a small amount of an vantoxidant such as 2,6-di-tbutyl-p-cresol to the slurry prior to filtration. Usually an amount of from about 0.1 to 1% antioxidant, based on the weight of polymer, will suffice.

The invention is illustrated 'by the following examples in which parts and percentages are by weight unless otherwise specified.

3 EXAMPLES In these examples, propylene was polymerized by passing gaseous propylene into a liquid, saturated aliphatic hydrocarbon diluent having .a boiling range of 170 to 200 C. containing titanium trichloride in the amount of 10 mM. per liter and diethylaluminum chloride in the amount of 20 mM. per liter of liquid. The polymerization was carried out for 6 hours at 50 C. under 28 p.s.i.g. of propylene.

At the end of the polymerization there was present in the polymerizer a slurry of fine particles of stereoregular polypropylene in the polymerization diluent, the polypropylene constituting about 35% of the slurry.

To the slurry there was added a predetermined amount of an alkanol under an atmosphere of nitrogen and the slurry stirred for one to two hours at 80 C. There was then added to the slurry a predetermined amount of a phosphate ester. (In some cases several minutes subsequent to the addition of the phosphate ester propylene oxide in the amount of 6.7 ml. per liter of diluent was added.)

A portion of the slurry was removed from the polymerizer, stirred in air for 10 minutes, filtered, and the filter cake washed with n-heptane at 70 C., and the n-heptane removed by drying. The portion of the slurry remaining in the polymerizer was filtered under nitrogen without exposure to air, and the filter cake washed in the same manner that the filter cake from the air-exposed slurry was washed. The following table presents the data obtained from several polymerizations carried out in the manner described and also control poly'merizations in which anionic phosphate ester was not added to the slurry.

exposed to air for 20 minutes. The results are as follows:

Phosphate Example C shows the ineffectiveness of tertiary phosphates in the process of the invention.

The process of the invention may be applied to the polymer produced in the polymerization of any l-olefin, e.g., ethylene, propylene, butene-l, 3-methylpentene-1, etc., or copolymers of such olefins, using the aforesaid catalyst system. In accordance with this known polymerization method, the olefin is contacted at relatively low pressure and temperature with a catalyst prepared by mixing a compound of a transition metal of Groups IV-B or V-B of the Periodic Table with an organornetallic compound of a metal of Group III-A of the Periodic Table. The soealled transition metal compound may be an inorganic salt such as a halide, oxyhalide, etc., or an organic salt or complex such as an acetylacetone, etc. Exemplary of the transistion metal compounds that may be used are titanium and zirconium tetrachlorides, titanium diand triehloride, tetrabutyl titanate, zirconium acetylacetonate, vanadium oxyacetylacetonate, etc. The organometallie compound that is reacted with one of the transistion metal compounds or mixtures thereof may be, for example, triethylaluminum, tripropylaluminum, triisobutylaluminum, tri- TABLE Propylene Phosphate Polymer analyses (parts per million) Example Alcohol (percent of oxide Phosphate ester N o. diluent) (percent of ester (percent of N2 Air diluent) diluent) Ti Al Ti A1 A n-Butanol (3) None 20 52 150 200 B Isopropanol (2) 0.67 18 27 100 140 1 n-Butanol (3) 0. 67 0.8 12 17 12 21 2 Isopropanol (2)- 1 0. 67 0. 8 10 19 18 24 3 n-Butanol (3) None Type II-.. 0.7 20 31 18 21 0. 67 Type IV 0.7 11 14 15 18 None Type III 0. 4 12 37 10 46 1 Added prior to phosphate ester.

I=Mixed monoand dilauryl phosphates. II=Di(tetramethylbutylphenyl) phosphat III=Technical di-n-butyl posphate IV =Diisooctyl phosphate.

As seen from the data in the table, when the phosphate I ester is omitted as in Controls A and B, the portion of the slurry that is worked up in the presence of air yields a polymer of unacceptably high metal content, although the portion of the slurry worked up under nitrogen gives acceptably low metal content. By comparing Controls A and B, it is seen that the addition of propylene oxide has little or no effect on reducing the metal content of the polymer worked up in the presence of air; it does, however, reduce the acidity of the slurry and renders it less corrosive to metal apparatus. In Examples 1 to 6 the addition of various phospate esters in accordance with the invention is shown to result in polymers having low ash contents, even when the workup is carried out in the presence of air.

In additional examples, the effect of various phosphates on hydrocarbon solutions of catalyst was studied to show the effect of the anionic phosphate ester in preventing precipitation of catalyst in the presence of air. In these examples, 6 mM. of diethylaluminum chloride and 3 mM. of TiCl were suspended in 300 ml. of the polymerization diluent as above described and to this was added 3% nbutanol. A clear blue solution resulted. Upon exposure of a portion of the solution to the atmosphere, a heavy white precipitate formed within a few minutes. To other portions of the solution each of the three phosphates mentioned below was added, and the solution allowed to stand e. di-n-butyl phosphate and 45% mono-n-butyl phosphate).

octylaluminum, tridodecylaluminurn, dimethylaluminum chloride, diethylaluminum bromide, diethylaluminum chloride, ethylaluminum dichloride, the equimolar mixture of the latter two known as aluminum sesquichloride, dipropylaluminum fluoride, diisobutylaluminum fluoride, diethylaluminum hydride, ethylaluminum dihydride, diisobutylaluminum hydride, etc., and complexes, as, for example, sodium aluminum tetraethyl, lithium aluminum tetraoctyl, etc.

Another method of carrying out the polymerization process is to use a two-component catalyst system. In one such system the insoluble precipitate which is formed by mixing the transition metal compound and the organometallic compound as described above is separated and then used in combination with an additional organometallie compound. The insoluble reaction product will be readily separated, if the reaction took place in an inert diluent, from the diluent and soluble reaction byproducts by centrifuging, filtering, or any other desired means. In some cases it may be desirable to wash the insoluble reaction product with additional amounts of hydrocarbon diluent in order to completely remove all of the soluble byproducts. This hydrocarbon-insoluble reaction product is then used in combination with an organometallic compound as exemplified above. This second catalyst component may be the same organometallic compound that was used in preparing the insoluble reaction product catalyst component or it may be a different organometallic compound. Of particular importance is the use of such a hydrocarbon-insoluble reaction product in combination with an aluminum trialkyl such at triethylaluminum, triisobutylaluminum, trioctylaluminum, etc.

In another two-component catalyst system, the whole reaction mixture formed on mixing a transition metal compound and an organometallic compound may be used in combination With an additional organometallic compound, if the latter is halogen-free. This two-component catalyst system is particularly useful for the polymerization of linear l-olefins.

These polymerization processes are carried out in a wide variety of ways, as, for example, as batch or continuous operations and with or without the use of an inert organic diluent as the reaction medium. However, for the purpose of the present invention there must be used an inert, organic diluent that is liquid under the reaction conditions. Preferred diluents are saturated aliphatic hydrocarbons boiling in the range of 70-250 C.

As pointed out already, the transition metal compound and the organometallic compound may be reacted in situ, as, for example, in the particularly effective method of polymerizing diolefins wherein a trialkylaluminum is re acted in situ with a tetraalkyl titanate. They may also be reacted prior to the introduction of the olefin or they may be reacted and then usedin combination with additional organometallic compound. They may also be added in increments during the polymerization and many other such variations may be utilized. Many other variations may be made in the polymerization system to which the process of this invention may be applied. For example, when lower molecular weight polymers are desired, a viscosity reducing agent such as a haloalkane, as, for instance, carbon tetrachloride, etc., or hydrogen, or other such agent may be added.

According to the invention, the first step in working up the slurry to recover high quality polymer is treatment with an alkanol, the purpose of this step being to solubilize the catalyst residues and thus render them more easily removable in subsequent steps. The alkanol used in this step is preferably one of those in the methanol to dodecanol series including branched chain alkanols, e.g., ethanol, propanol, isopropanol, n-butanol, sec-butanol, t-butanol, isobutanol, n-octanol, 2-ethylhexanol, n-decanol, etc.

The amount of alkanol added at the end of the polymerization may be varied widely and will depend chiefly upon the amount of catalyst used in the polymerization and other reaction conditions, but in general is from about 0.1% to about 10% of the volume of the polymer slurry, and preferably is from about 1% to about 5%. Much larger quantities may be used but are not believed to serve any useful purpose and hence are not usually desired for practical considerations. The alkanol treatment may be carried out at any desired or practical temperature, as may be seen from the above examples, but generally will be from about C. to about 100 C., and preferably from about 25 C. to about 80 C. Only a short holding time is required for the alkanol treatment, depending upon the temperature, catalyst, etc. -In general, a period of from about minutes to 1 hour is adequate, but any longer period of time is practical, as for example, overnight, etc. The alkanol treatment is carried out in an inert atmosphere, i.e., in the absence of oxygen, Water, etc., and hence an oxygenand water-free atmosphere is used, as for example, nitrogen or other inert gas.

The process as it has been described up to this point is well known in the art. At this point, however, the invention departs from the art by the addition of a small quantity of an anionic, oil-soluble phosphate ester as previously described. This ester can be added While the slurry is at a temperature ranging from about 25 to 80 C. The amount of phosphate ester can vary from about 0.1 to about 2.0% by weight of the diluent. Typical anionic,

oil-soluble phosphate esters which can be used in the process of the invention include monoalkyl and dialkyl esters or orthophosphoric acid in which the alkyl radical(s) contains from about 4 to 22 carbon atoms. Typical esters of this class are di-n-butyl phosphate, monolauryl phosphate, dilauryl phosphate, diisoamyl phosphate, diisooctyl phosphate, di 2 ethylhexyl phosphate, monostearyl phosphate, and mixed esters such as isoamyl isooctyl acid orthophosphate, ethyl oleyl acid orthophosphate, ethyl lauryl acid orthophosphate, and the like.

Following the addition of the phosphate ester, the slurry is next simply agitated for a period of time ranging from about 5 to 60 minutes, and by virtue of this treatment acquires relative insensitivity to air and moisture while the catalyst residues remain in solution.

It is now possible to separate the polymer from the slurry in the presence of air, even very humid air. Separation can be effected by filtration, centrifugation, or any equivalent technique. The elimination of the need for carrying out the separation in the absence of air is a remarkable step forward in the art since it eliminates the cumbersomeness and expense of maintaining an inert atmosphere.

Following separation of the polymer, it is simply washed with an inert liquid hydrocarbon. Any liquid hydrocarbon or mixture of such hydrocarbons may be used for the washing, as, for example, aliphatic hydrocarbons such as n-hexane, n-heptane, and aromatic hydrocarbons such as benzene, toluene, and xylene, and cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane, and the like. For practical reasons, the washing is usually carried out at room temperature but may be carried out at elevated temperatures up to about C., if desired.

Obviously, many variations may be made in the purification process of this invention. One such variation, which has been illustrated in the examples, comprises the addition of propylene oxide in the amount of about 4 to 8 ml. per liter of diluent just prior to addition of the phosphate ester. This addition of propylene oxide neutralizes any HCl that is present in the slurry and makes it possible to carry out subsequent steps in metallic apparatus that would otherwise be corroded.

In order to remove the hydrocarbon washing liquid from the hydrocarbon, a simple drying operation is adequate in most cases. In other cases, it may be desirable to subject the polymer to a steam distillation treatment before drying in order to remove all traces of the hydrocarbon liquid.

What I claim and desire to protect by Letters Patent is:

1. In the process for removing catalyst residues from a polyolefin which has been obtained by polymerizing an olefin in the presence of a catalyst comprising a compound of a metal selected from the group consisting of the metals of Groups IV-B and V-B of the Periodic Table in combination with an organometallic compound of .a metal of Group II I-'-A of the Periodic Table in the presence of an inert liquid diluent wherein the slurry of solid polymer in diluent that is obtained from the polymerization is treated with an alkanol to solubilize the catalyst residues, following which the polymer is separatedfrom the slurry and washed with an inert liquid hydrocarbon, the improvement which comprises adding to the alkanoltreated slurry prior to separation of the polymer an anionic, oil-soluble phosphate ester having the formula in which R represents hydrogen or a hydrocarbon radical of 4 to 22 carbon atoms and R represents a hydrocarbon radical of 4 to 22 carbon atoms in the amount of about 0.1 to 2% by weight of the diluent to render the slurry insensitive to air and moisture.

2. The process of claim 1 in which the polyolefin is polypropylene.

3. The process of claim 1 in which the polyolefin is polyethylene.

4. The process of claim 1 in which R and R are each 5 7. The process of claim 1 in which the slurry is treated I with propylene oxide to neutralize any hydrogen chloride present therein.

8 References Cited UNITED STATES PATENTS 3,308,105 3/1967 Hoyt 26093.7

JOSEPH L. SCHOFER, Primary Examiner.

L. EDELMAN, Assistant Examiner.

U.S. Cl. X.R. 

